Separation of sultaines by dilution



c. M. CAMBRE ETAL 3,227,749

SEPARATION OF SULIAINES BY DILUTION Jan. 4, 1966 Filed Sept. 15, 1964 Temperature Meliing Point Tw T0 Surfactant in Water Cushmcm M. Cumbre Kennerh W. Herrmonn INVENTORS United States Patent SEPARATION OF SULTAINES BY DHUTEON Cushrnan Merlin Camhre and Kenneth Walter Herrrnann, Cincinnati, Ohio, assignors to The Procter 8:

Gamble Company, Cincinnati, Ohio, a corporation of Ohio Filed Sept. 15, 1964, Ser. No. 398,151 3 Claims. (Cl. 260-491) This application is a continuation-in-part of our copending application relating to Detergent Purification, Serial NO. 216,118, filed August, 10, 1962, now abancloned.

This invention relates to the purification of detergent surfactants. Specifically it relates to the separation of certain specific detergent surfactants from accompanying electrolytes which act to solubilize these specific detergent surfactants.

The production of detergent surfactants is quite often accompanied by the production of inorganic salts which constitute an impurity. This impurity may be acceptable or even desirable under certain circumstances, but under other circumstances such as in the formulation of liquid detergent compositions or dentifrices, these inorganic impurities may be undesirable. Such electrotyte impurities may be undesirable in liquid compositions because of the premium on weight and the desirability of using another functional electrolyte in formulating a liquid detergent.

Accordingly, it is an object of this invention to provide a process for separating certain detergent surfactants from electrolyte solutions.

It is a further and more specific object of this invention to provide a process for separating certain detergent surfactants from inorganic salts.

The sole figure is a representative phase diagram for the detergent surfactants utilized in the process of this invention as hereinafter more fully discussed.

The specific detergent surfactants which can be purified by the process of this invention have the general formula wherein R and R are selected from the group consisting of hydrogen, methyl and ethyl groups; X is selected from the group consisting of hydrogen and hydroxyl groups; and R is an alkyl chain of (A) from about 14 to about 18 carbon atoms when X is a hydroxyl group and R and R are alkyl chains having from one to about two carbon atoms; (B) from about 12 to about 18 carbon atoms when R and R are hydrogen atoms and X is selected from the group consisting of hydrogen and hydroxyl groups; and (C) from about 16 to about 20 carbon atoms when X is a hydrogen and R and R are alkyl chains of from one to about two carbon atoms. (These detergent surfactants will be referred to hereinafter as sultaine surfactants.) These sultaine surfactants are separated from an aqueous electrolyte solution by dilution with water as hereinafter more fully described.

These particular sultaine surfactants are unique in that the addition of an electrolyte to their aqueous solution lowers their Krafft temperature. This Kr-afft temperature, as employed herein, is a narrow range of temperatures for a given compound within which a small increase in temperature gives a very large increase in solubility of the compound. Lowering of the Kraift temperature for a given sultaine-electrolyte-water system has the practical effect of raising the solubility of the surfactant in water for any temperature above the temperature to which the Krafft temperature is lowered and below the Kralft temperature of the surfactant in water (without electrolyte).

Normally, the addition of an electrolyte to an aqueous anionic surfactant solution will raise this Krafft temperature (lowering practical solubility), but the addition of an electrolyte to the sultaine surfactants utilized in the process of this invention lowers their Kratft temperature (increases practical solubility). Sultaine surfactants with shorter chain lengths than the surfactants of this invention have Kratft temperatures below the freezing point of water so that dilution with water will never precipitate them. Sultaine surfactants with longer chain lengths are too insoluble to be solubilized by electrolytes.

The amount of lowering of the Krafit temperature is dependent upon the type and amount of electrolyte present. Although it is not desired to be bound by theory, it is believed that electrolytes having an anion with relatively low charge density (ratio of number of charges in the anion to volume of the anion) are most effective in lowering the Krafft temperature. Examples of electrolytes which are particularly effective in this respect include potassium and sodium benzoates; 2-hydroXy-3-chl0ropropane sulfonic acid sodium salt; 2,3-dihydroxy propane sulfonic acid sodium salt; 2-hydroxy-l,3-propane disulfonic acid disodium salt; and alkali metal, ammonium, and substituted ammonium chlorides, bromides and iodides such as sodium chloride, potassium chloride, sodium bromide, potassium bromide, sodium iodide, ammonium chloride and triethanol ammonium chloride. Other particularly effective anions include nitrates, nitrites, borates, thiocyanates, bisulfates, bisulfites, chlorates, perchlorates, bromates, iodates, periodates, acrylates, crotonate, sorbates, phenyl acetates, naphthates, cinnamates, lactates, acetates, 3,5-dinitro benzoates, p-chlorobenzoates, o-nitrobenzoates, isobutyrates, isovalerates, trimethyl acetates, ethyl benzene phosphonates, furoates, and tetra phenyl boron. Examples of less effective electrolytes include many of the common ingredients found in deter gent compositions such as potassium and sodium pyrophosphates, sodium and potassium tripolyphosphates, potassium and sodium silicates, potassium and sodium sulfates, potassium and sodium ethylenediamine tetraacetates, and sodium and potassium nitrilo triacetates.

The effect of this lowering of the Krafft temperature is such that a simplified method of separation of sultaine surfactant from electrolyte is possible. To explain this method, reference is made to a typical phase diagram, FIG. 1, in which the crystalline solubility (to left of point A) of a typical sultaine surfactant of this invention in water is plotted as a function of temperature with a solid line and the crystalline solubility (to left of point A of the same typical sultaine surfactant in an aqueous electrolyte solution of constant concentration is plotted as a function of temperature with a broken line. It will be understood that the position of this broken line varies with the concentration of electrolyte in water and that there is a series of these dotted curves for different electrolyte concentrations. In general, the more electrolyte that is present the lower the broken line curve is on the diagram. The portion of the phase diagram to the right of A and A is highly simplified, In general, it can be said that the process of this invention is primarily concerned with the portion of the phase diagram to the left of A and A but it is possible and often desirable to start with a system (e.g., a surfactant paste) on the right side of the diagram and by diluting the system with water change to the left side of the diagram.

The lines on the diagram represent the boundaries for homogeneous solutions. Region B is homogeneous and is Commonly referred to as nigre; regions C, C D, and D are characterized by the presence of a mesomorphic phase; region B is characterized by the presence of crystals of surfactant and surfactant solution.

The method operates between the Kratft temperature for the sultaine surfactant in water (T and the Kratft temperature of the sultaine surfactant in the electrolyte solution (T Dilution of a concentrated sultaine paste or solution containing electrolyte with water has two effects. The first is to lower the concentration of the sultaine surfactant. The second effect is to lower the concentration of the electrolyte which in turn results in a rise in the Kraflft temperature. If this second effect is sufficient to raise the Krafft temperature above the process operating temperature (T then the essentially pure sultaine surfactant will precipitate. Filtration or settling plus decantation will then separate the supernatant liquid from the precipitate. It is possible, in some instances, to precipitate the sultaine surfactants of this invention from electrolyte solutions simply by cooling the sultaine solution, but this is expensive in most cases and impossible in some cases where the electrolyte present has lowered the Kraift temperature to a point where the water crystallizes before the sultaine surfactant. Normally, cooling alone is not as eflicient as dilution alone or a combination of cooling and dilution. Normally, there will be a combination of steps which is most efficient or desirable depending upon the particular sultaine-water system and economic considerations including such things as cost of separation and cost of cooling.

Normally, this process is of value when the concentration of the sultaine surfactant in water is above about .5 by weight of the electrolyte solution initially. Also, this process is practicable only when there is sufficient electrolyte present to lower the Krafit temperature of the sultaine surfactant at least about C., which is normally at least V2 by weight of electrolyte in the solution initial- 1y. If there is more than enough electrolyte present to saturate the solution it should be removed prior to dilution. It is necessary, of course, that the electrolyte concentration must also be sufficient to lower the Krafft temperature of the sultaine surfactant solution below the process operating temperature in order that the sultaine surfactant can be put in solution at a reasonable concentration if it is not initially in solution. This operating temperature is normally from about 1 C. above the freezing point of the solution to about 1 C. below the boiling point of the solution, preferably from about C. to about 60 C. depending on the Krafft point in water of the sultaine surfactant and the economics involved. The amount of dilution with water necessary to precipitate the sultaine surfactant is normally not over 100-1 on a volume basis and will vary with the initial concentrations of sultain surfactant and electrolyte. Ordinarily, a dilution of at least about 1.1-1 on a volume basis is needed to effect a precipitation of the sultaine surfactant from solution. The diluted mixture can be aged or cooled, if desired, to aid in recovery of the maximum amount of sultaine surfactant.

The method of separation of the precipitate is, in general, not critical. However, economic considerations may make this step very important. The nature of the precipitate is such that it will quickly clog a filter so that care must be taken to select filtration equipment which will not require the buildup of a large layer of residue for economical operation. In many instances it will be most advantageous to centrifuge the diluted suspension or allow the precipitate to settle and then deeant the supernatant liquid.

Krafft Temperature as used herein is a well known term. (See, e.g., Surface Chemistry, Osipow, published by Reinhold Publishing Company, N.Y., 1962.) These Krafft temperatures may be determined quite simply by an experimental method similar to that described in The Journal of the American Chemical Society, volume 73, p. 3350, July 1951, by Broome et al. One simply heats and cools solutions of the surfactant, either with or without electrolyte being present, and determine visually that point at which crystalline surfactant separates by the appearance or disappearance of cloudiness. The Krafft temperatures given in the following example were determined in this manner.

The following example illustrates the practice of this invention.

Example A solution of 1% by weight 3-(N,Ndirnethyl-N-hexadecyl ammonio)-2 hydroxy propane-l-sulfonate and 1% by weight sodium chloride in water was prepared. The Krafft temperature of the 3-(N,N-dimethyl-N-hexadecyl ammonio)-2-hydroxy propane-l-sulfonate in water is about C. and its Krafft temperature in the 1% NaCl solution was about 38 C. The solution was diluted 50:1 with water at an operating temperature of about 60 C. The 3-(N,N-dimethyl-N-hexadecyl ammonio)-2-hydroxy propane-l-sulfonate precipitated (indicating that the Krafit temperature of the surfactant in the diluted solution was above the operating temperature of 60 C.) and was separated from the aqueous solution by filtration to give a yield of about 96% of the product originally in solution, said separated product containing only 0.2% NaCl by weight.

What is claimed is:

1. A process for separating sultaine surfactant selected from the group consisting of (a) those having the formula wherein R is an alkyl chain of from about 14 to about 18 carbon atoms and R and R are alkyl chains having from one to about two carbon atoms; (b) those having the formula wherein R is an alkyl chain of from about 16 to about 20 carbon atoms and R and R are alkyl chains having from one to about two carbon atoms; and (0) those having the formula wherein R is an alkyl chain of from about 12 to about 18 carbon atoms and X is selected from the group consisting of hydrogen and a hydroxyl group, from an aqueous electrolyte solution wherein said electrolyte is selected from the group consisting of: potassium and sodium benzoates; 2-hydroXy-3-chloro-propane sulfonic acid sodium salt; 2,3-dihydroxy propane sulfonic acid sodium salt; 2- hydroXy-1,3-propane disulfonic acid disodium salt; alkali metal, ammonium and substituted ammonium chlorides, bromides and iodides; nitrates; nitrites; borates; thiocyanates; bisulfates; bisulfites; chlorates; perchlorates; bromates; iodates; periodates; acrylates; crotonates; sorbates; phenyl acetates; naphthates; cinnamates; lactates; acetates; 3,5-dinitro benzoates; p-chlorobenzoates; o-nitrobenzoates; isobutyrates; isovalerates; trimethyl acetates; ethyl benzene phosphonates; furoates; tetra phenyl boron; potassium and sodium pyrophosphates; sodium and potassium tripolyphosphates; potassium and sodium silicates; potassium and sodium sulfates; potassium and sodium ethylenediamine tetraacetates; and sodium and potassium nitrilo triacetates; and wherein the concentration of the electrolyte is above about /2% by weight of said solution and sufficient to lower the Krafl't temperature of said surfactant at least 5 C. below the Krafft temperature of said surfactant in water and below the operating temperature of the process, and the initial concentration of said surfactant is above about .5 by weight of said solution, said process comprising the steps of diluting the surfactant-electrolyte solution with water sufficient to prewherein R is an alkyl chain of from 14 to 18 carbon atoms, and the electrolyte is sodium chloride.

3. The process of claim 1 wherein the operating temperature is from about 10 C. to about 60 C.

References Cited by the Examiner UNITED STATES PATENTS 2,375,164 5/1945 Bennett 260-501 2,519,573 8/1950 Hogan 260501 2,697,116 12/1954 Stayner 260-501 2,795,603 6/1957 Sietserma 260501 OTHER REFERENCES Weissberger: Techniques of Organic Chemistry, vol. 1H, part I, 1956, pp. 304, 472, 473.

Moilliet et 211.: Surface Activity, 1961, pp. 44. 45, 485, 486.

5 LORRAINE A. WEINBERGER, Primary Examiner. 

1. A PROCESS FOR SEPARATING SULTAINE SURFACTANT SELECTED FROM THE GROUP CONSISTING OF (A) THOSE HAVING THE FORMULA 